Happy Sunday
@Mossy, and thanks for the earlier points. I hope you have had a great weekend so far.
because without it, a shaft of that strength could whip and cause vibration if it was a one piece. A stiffer shaft material could avoid this, but likely with more weight, and all vehicle manufacturers have been trying to avoid unnecessary weight in order to reduce fuel consumption. The need for the middle bearing is not directly related to the horsepower, it is more a matter of shaft stiffness and rpm.
Mind you, I am not an engineer, so that's just my opinion. OTOH, I have been down the shaft vibration thing in a big way with a boat I once owned, so I have had to do my homework. Shaft vibration can get violent in a heartbeat. When it started on the boat, it was instant, and shook the entire boat. The diesel was 30 hp.